The research and development of new MEMS technologies and electronic materials for the detection of toxic and explosive vapors, with high sensitivity and selectivity is of utmost importance for many commercial, environmental, security applications and for US military missions.
Detection techniques include a variety of physical and chemical methods related to changing the output signal under exposure to target analytes. Physical methods include: nuclear quadrupole resonance, ion mass spectroscopy, gas chromatography, X-ray diffraction, electron capture detection, and laser photofragmentation. These techniques are selective enough, but can be expensive, bulky and cannot be employed for real-time, fast and remote analyte detection. Also, most of the chemical sensors have been studied, developed, and fabricated in the macro format using traditional techniques for the deposition of sensory polymers (spin-casting, coating, spraying) onto relatively large area substrates followed by coupling to a separated detection/acquisition system (Walt et al, Chem. Rev. 100: 2595, 2000; Grate, Chem. Rev. 100: 2627, 2000). Such devices could be employed for pattern recognition of vapor mixtures. However, vapor concentration should be high enough to prevent a false response and to correctly identify the explosive chemical signature. Since many toxic and explosive vapors (for example, TNT, RDX, PETN) are related to low pressure vapors, the critical issue becomes an enhancement of sensor sensitivity and selectivity to provide a fast, real-time response with a minimum false alarm.
Porous silicon (PSi) has been extensively studied for a number of semiconductor applications since it was discovered in the late 1950's. More recently, PSi has been shown to exhibit visible luminescence (Canham, Appl. Phys. Lett. 57:1046; 1990), suggesting promising applications in silicon-based optoelectronic devices. Other porous semiconductor materials such as gallium arsenide, for example, have also been studied to a lesser extent (Schmuki et al, Appl. Phys. Lett. 72:1039; 1998).
U.S. Pat. Nos. 5,338,415; 5,453,624; and 6,720,177 (Sailor et al) describe a method for the detection of chemicals by reversible quenching of PSi photoluminescence and a device for the detection of organic solvents by PSi photoluminescence, respectively. A silicon wafer was electrochemically etched (anodization) with a 50:50 ethanol/hydrofluoric acid (HF) solution to produce a PSi wafer. When the PSi wafer was illuminated with a laser light source in the presence of an organic compound, such as tetrahydrofuran (THF), diethyl ether, methylene chloride (MeCl.sub.2), toluene, o-xylene, ethanol and methanol (MeOH), the inherent luminescent emission intensity of the PSi was significantly decreased (i.e., the photoluminescent response of the PSi was quenched). In these patents it is noted that the transduction mechanism in the above patents is the luminescence caused by porous Si only.
Lin et al. describe a biosensor based on induced wavelength shifts in the Fabry-Perot fringes in the visible light reflection spectrum of a thin flat film of PSi (Science 278:840; Oct. 31, 1997). Optically flat thin films of PSi, prepared by electrochemical etching are sufficiently transparent to display Fabry-Perot fringes in their optical reflection spectrum. A recognition element is immobilized on the flat PSi film. Subsequent binding of an analyte to the recognition element therefore results in a change in the refractive index of the PSi film and is detected as a corresponding shift in the interference pattern. Janshoff et al (J. Am. Chem. Soc. 120:12108; 1998) also describe the PSi for biosensor applications utilizing a shift in a Fabry-Perot fringe pattern, created by multiple reflections of illuminated white light on the air/PSi layer and PSi/bulk silicon interface, as a means for detecting molecular interactions of species in a solution with immobilized ligands as receptors.
U.S. Pat. No. 6,780,649 (Armstrong et al) describes the PSi layer modified with recognition elements. A PSi layer has its own photoluminescence (PL). A PSi modified with such recognition elements can interact with a target analyte so that a wavelength shift and/or change in PL intensity. Thus, transduction mechanism in these sensors is photoluminescence of PSi, but not of the sensory element itself.
U.S. Pat. No. 7,226,733 (Fauchet et al) describes a biological sensor comprising of a porous semiconductor structure including strata of alternating porosity; and one or more probes coupled to the porous semiconductor structure. The probes that are binding to a target molecule result in change in a refractive index of the biological sensor upon binding of more probes to the target molecule. This description also includes an emission pattern which shifts following exposing, whereby a shifted photoluminescent emission pattern indicates the presence of the target molecule in the sample. However, this invention is related to only the microcavity biosensors “which are useful for identifying the presence of a biological target molecule or organism in a sample, as well as methods for making such microcavity biosensors and their use”. In addition, as it follows from the detailed description, emission of the sensors comes only from the porous Si, but not from the organic sensory material entrapped inside PSi pores
The series of articles and patents by Swager et al (J. Am. Chem. Soc. 120: 11864, 1998; U.S. Pat. No. 7,208,122) propose a new concept, namely the “molecular wire” approach, related to emissive optochemical sensors for the detection of explosive vapors. The major issue here is the amplification mechanism based on an energy migration effect allowing very high device sensitivity, which is of utmost importance for the detection of explosives with a low pressure of saturated vapors. U.S. Pat. No. 6,686,206 and an article (J. Phys. Chem. B 106:8468, 2001) by Levitsky et al also describes the optochemical sensors involving amplification mechanism of luminescence, however, based on the direct Forster energy transfer. Despite high sensitivity, the above emissive sensors suffer low selectivity as quenching or enhancing of the emission demonstrates the similar behavior for different parts of the luminescent spectrum.
It would therefore be desirable to have material useful for detecting low pressure vapors, which combines the high sensitivity of the emissive sensory elements and dependence of the refractive index of the porous semiconductor on target molecules.